Throughout the 1990s the majority of internet users began their session with the noisy handshake of a dial-up modem, but what exactly was all that electronic chatter about? Read on as we investigate one of the more iconic sounds of the burgeoning Internet age.

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，它是一个由Q&a网站组成的社区驱动分组。

虽然拨号调制解调器的使用率可能会从上世纪90年代接近100%的市场饱和率下降到目前美国互联网用户的10%，但拨号调制解调器连接的声音仍然留在各地极客的记忆中。本周，我们将了解这一嘈杂过程背后的技术，以及您拨入internet会话时到底发生了什么。

问题

超级用户读者Celeritas提出了一个问题，多年来，数百万人肯定都在问自己：

I know that the signal was just tone pulses but why was it when (back in the 90s) when you first connected to the internet you heard a bunch of funny noises. After that if you were to use the internet, it still was using the telephone line, why no funny noises then?

为什么呢？吵闹的那部分发生了什么？为什么之后会保持沉默？

答案

几个超级用户贡献者为我们提供了一个具体的答案。斯科特·张伯伦写道：

Modems originally allowed you to send data over a network that was designed to only carry voice. Because of that, the communication method between two modems had to be in the audible hearing range (or it would not get carried on the phone line). This is no longer needed because the phone system can now carry both voice and data at the same time (DSL).

The sounds were there all the time, you just needed to pick up the phone to hear it. The reason they played it over a loudspeaker to start with is so you could hear if somthing went wrong with the connection (busy signal, wrong number, a person picked up instead of a modem on the other end, etc).

Tylerl对此进行了扩展，并解释了如何操纵调制解调器向下传输：

The whistles and chirps and buzzes that you hear when a modem is going through its initial handshake process is a test of the telephone line quality. A modem send precisely specified sounds and the other listens see what it actually hears on the other end. This way the modems know how clear the line is between them and what sort of frequencies they can use to communicate with each other. The more frequencies they can use and the lower the noise, the higher the speed they’ll be able to communicate at.

If a connection ever failed due to connection quality, it would generally fail during this initial handshake process. And if you were listening, you could usually tell why (e.g. you got an answering machine on the other end instead of a modem).

As such, modems were usually configured to play this handshake sequence out loud. This was configured by sending AT M1 to the modem during setup. Alternately, AT M2 means to leave the speaker on all the time, while AT M0 means don’t turn the speaker on at all. See the AT command set for more information.

The actual tran**ission noise that you would hear if you picked up the phone during an active session (as opposed to during this handshake procedure) just sounds like static.

哦，在M0的魔力；发现这个命令就像被赋予了隐形斗篷，每个人都在深夜偷偷浏览。Tylerl指出，高波特率的流量听起来就像静态的，而Supercat指出，非常低的波特率调制解调器则是另一回事：

At 300 baud, it’s possible to audibly hear incoming data. On occasi***, I’ve turned on the modem speaker if I wanted to hear when characters arrived on a generally-idle line. Higher baud rates use a “data-scrambler” circuit so that most patterns of data are no longer audibly distinguishable.

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